Chiang Rai Governor Thanin Supasean, who is studying the situation, handed out masks Thursday.
For people to remain in sound health, each cubic metre of air should contain no more than 120 micrograms of small dust particles, which are PM10 in size.
Once the amount of dust particles goes beyond 200 micrograms, people are usually advised to stay indoors. However, the situation in Mae Sai was clearly worse.
The Nation